A commonly known multilayer polyolefin separator membrane, which may be used in various lithium batteries, such as, for example, a lithium ion rechargeable battery, may include three layers of a polyolefin separator membrane configured, sometimes, as trilayer polypropylene/polyethylene/polypropylene (PP/PE/PP), where an inner polyethylene (PE) microporous membrane is sandwiched between two outer layers of polypropylene (PP) microporous membrane. Polypropylene, when used as a monolayer microporous separator membrane or when used as the outer layers in a multilayer microporous separator membrane may, in some instances, have a higher pin removal force than a polyethylene (PE) layer due to a higher surface coefficient of friction of the PP separator or outer layer.
U.S. Pat. No. 6,692,867 discloses the addition of a metallic stearate additive that may be added to a polypropylene resin in the production of a monolayer polypropylene microporous separator membrane or in the production of one or more outer polypropylene layers in a multilayer microporous separator membrane as a means to lower the pin removal force. One example of a metallic stearate may be calcium stearate. In some instances, calcium stearate may lower the pin removal force of a polypropylene microporous separator membrane when at least 50 parts per million is added to a polypropylene resin during the melt extrusion process to form the separator membrane.
The use of polypropylene as the outer layers in a multilayer (for example, trilayer) battery separator microporous membrane may, in some instances, be a preferred configuration. U.S. Patent Publication No. 2007/0148538 proposes various multilayer configurations, which may include “polypropylene/polyethylene/polypropylene” (PP/PE/PP) as a microporous shutdown trilayer separator membrane where the higher tensile strength, higher melt temperature polypropylene may be used as the outer exterior layers and a low melt temperature polyethylene may be used as the inner, thermal shutdown layer. U.S. Pat. Nos. 5,952,120, 5,691,077, and 8,486,556 and U.S. Patent Publication Nos. 2014/0079980, and 2008/0118827 disclose various methods for making multilayer microporous battery separator membranes.
Polypropylene, in some instances, may have a higher pin removal force or coefficient of friction than polyethylene that may result in pin design or pin removal issues during the production of certain cylindrical, jellyroll or prismatic type batteries or cells. In the manufacture of cylindrical and prismatic batteries, one important step in the battery cell preparation process may be the winding step where an “electrode/separator/electrode” stacked and layered assembly is wound around a central winding pin at a very high speed. When a separator membrane has a high pin removal force, a problem may occur in some instances where one or more layers of the electrode/separator/electrode layered assembly may be dislodged as the central winding pin is retracted and removed from the tightly wound up electrode/separator/electrode stacked assembly. One or more inner wraps of a battery separator membrane may dislodge or telescope out from the electrode/separator/electrode wound assembly as the winding pin is removed, which may, in some instances be caused by an unacceptably high level of pin removal force. A battery separator membrane, which may have an unacceptable level of pin removal force, may require pin redesign or may not allow for the economic production of uniform and perfectly wound cylindrical or prismatic battery cells.
There is a need for a microporous multilayer separator membrane that contains one or more polypropylene layer(s) and polyethylene layer(s) and that has good tensile and/or mechanical strength and that has low pin removal force that may enable high production yield rates in a battery cell winding process.